Fig. 1

Three radiographic images of proximal femurs at different degrees of external rotation, with a schematic view of their orientation in the transverse plane. The offset measured on the radiographs diminishes as the angle of external rotation increases, while the true offset remains the same. The vertical yellow arrows indicate the position of the radiographic lines, which are not visible on the right-hand image because the rotation is too extreme. The two red squares on the schematic images represent the physical location of the radiographic lines. The left-hand image demonstrates our hypothesis that the true offset can be measured radiographically when the radiographic lines appear superimposed